Tests for horse meat by Compass Group, Whitbread, and a county school meals service have come back positive.
Compass Group has revealed that it had discovered a minor amount of horse DNA in a beef burger product sourced from Rangeland Foods in Ireland, despite the manufacturer's assurances that affected ingredients had not entered the food chain.
"Despite receiving written assurances from Rangeland Foods that none of the identified horse DNA had entered its production, we immediately took the precautionary measure of withdrawing this product and we stopped any further purchases.
"We subsequently carried out a DNA test on a sample of the withdrawn product, which identified a minor amount of horse DNA," a spokesperson for Compass Group said in a statement.
"This is totally unacceptable. We have informed all of the affected sites of these developments, explained the actions we have taken and issued unreserved apologies."
Compass said it is working with the Food Standards Agency and the Food Safety Authority of Ireland to investigate how the situation arose and ensure it doesn't happen again.
Hotel and restaurant group Whitbread, one of the UK's largest hospitality companies, has found horse meat in two of its products: beef lasagne and beef burgers.
The company said the two products were sold in its Premier Inn, Brewers Fayre, Beefeater Grill and Table Table brands.
A spokesperson from Whitbread said the company is "shocked and disappointed at this failure of the processed meat supply chain".
"As an industry it is clear we need the supply chain to deliver products to the highest standards of food integrity and quality that we and our customers expect.
"As a responsible business we shall work with the FSA to implement a robust testing regime to avoid this happening in the future. We would like to sincerely apologise to our customers for any concerns or inconvenience that this may cause."
Meanwhile cottage pie delivered to 47 Lancashire schools has also tested positive for horse DNA. The products have since been removed from all the school kitchens.
"Lancashire County Council has withdrawn a beef product from school kitchens after it provisionally tested positive for traces of horse DNA," the council said in a statement.
"The provisional results of the tests on a pre-prepared cottage pie from an external supplier were reported late yesterday evening, 14 February, and have been passed onto the Food Standards Agency."